Four-hundred workers at Holden's Elizabeth car plant in Adelaide have taken voluntary redundancies and will finish work on Friday.

The future of Holden in Adelaide is still in doubt with the company expected to finalise its business plan in September.

Federation of Vehicle Industry Unions spokesman John Camillo says more than 500 workers put their hands up to take a separation package.

"It's a better situation than the situation of people being forced out the door with no choice whether they want to stay or not so, it's a good outcome for all, but it's a sad day for those 400 people," he said.